1. Field of the Invention
The present invention relates to the field of process chemical baths, particularly hydrogen peroxide containing baths, and more particularly, to a scheme for monitoring and controlling aqueous baths which are utilized for semiconductor manufacturing.
2. Prior Art
Chemical solutions have been utilized extensively for the manufacture of semiconductor devices. Wet chemical processing baths have been used for cleaning semiconductor wafers, as well as for etching deposited films on these wafers. For example, the use of hydrogen peroxide (H.sub.2 O.sub.2) containing solutions for cleaning silicon semiconductor wafers is well known. In addition to wafer cleaning, hydrogen peroxide is utilized in combination with sulfuric acid for photoresist removal and in combination with phosphoric acid, sulfuric acid or ammonium hydroxide for selective titanium etching.
It is known that to ensure uniform processing in advanced VLSI (very large scale integrated circuit) and ULSI (ultra large scale integrated circuit) manufacturing, it is critical to maintain a chemical composition of a bath at a specified concentration level. Alternatively, more uniform processing can be attained by measuring the solution concentration and adjusting the wafer processing time to compensate for changes in solution composition. Maintaining specified concentration levels is especially complicated in hydrogen peroxide based solutions. For example, in the case of an NH.sub.4 OH--H.sub.2 O.sub.2 --H.sub.2 O bath used for particle removal from wafers at elevated temperatures in a megasonic type bath, hydrogen peroxide decomposition to oxygen and water occurs rapidly and NH.sub.3 is lost from the bath due to its high vapor pressure. In other types of hydrogen peroxide baths, water loss by evaporation can lead to concentration changes.
At most semiconductor fabrication facilities, liquid processing baths are used for a certain time period and then discarded. This practice not only results in high chemical costs, but it also leads to the generation of more waste than would be required. Environmentally, it is preferred to reduce such waste.
In more advanced manufacturing facilities, automated controllers are utilized to achieve some degree of chemical composition control. These controllers spike the bath with certain chemicals at predefined intervals and can also add one or more chemicals to the bath to make up for a drop in the bath liquid level. With the exception of liquid level sensors, no analytical instrumentation is employed to provide feedback for guiding the chemical composition adjustment process. Thus, departures from "normal" operating conditions are not detected, nor are appropriate corrective actions taken.
Intermittent bath analysis can be performed by manually taking samples from the bath for purposes of analysis. This sample is then rushed to a laboratory for assay of the particular chemical of interest. A common chemical analysis technique utilizes a visual titration for H.sub.2 O.sub.2 using standardized permanganate solution. Often delays, in having the analysis performed, can lead to erroneous results. Safety issues and manpower constraints also combine to make periodic sampling of baths a rarity. Another deterrent to this method of bath monitoring is that the introduction of a sampling container into the bath, especially by a human operator, increases the chance of introducing contaminants into the chemical bath.
Although the methods of bath operation and testing outlined above have been adequate for manufacturing prior art semiconductor devices, such techniques are cumbersome and not reliable for providing substantially continuous and/or in-situ monitoring of a wet chemical bath.
In the manufacture of state-of-the-art and future generations of semiconductor devices, it is appreciated that the specified tolerances for chemical composition in such baths will require tighter tolerances. In order to manufacture even smaller submicron semiconductor devices, as well as improving the manufacturing yield, it is imperative that other schemes for maintaining a tighter control on the chemical make-up of a liquid processing bath is desired. In order to ensure uniform processing, such as uniform cleaning without surface damage, stripping and/or etching, it is imperative to continually monitor and, if necessary, appropriately adjust the concentration level of chemicals in an aqueous bath without human interaction.
The present invention provides for a scheme in which substantially continuous, frequently repetitive and/or in-situ monitoring of the chemical bath can be obtained and, further, such measured data can be analyzed and computed in order to automatically control the composition of a bath. In the case of H.sub.2 O.sub.2 baths, it is usually essential to measure and control more than one component at the same time. Moreover, this scheme provides for an analytical methodology which can be most easily implemented in a semiconductor fabrication facility with reasonable cost and a lower probability of introducing deleterious contamination within the facility. This scheme also provides for desired analytical accuracy.